Also known as the "Great Os", the O Antiphons are Magnificat antiphons used at Vespers during the last seven days of Advent. The exact origin off the Antiphons is not known, but it is commonly thought they were present during the sixth century when Boethius mentions them in his writings. By the eighth century the Antiphons were used widely both in Rome and in monasteries. 

 

They are commonly known as the O Antiphons because the title begins with the vocative particle "O" in each title and each Antiphon contains a name of Christ:

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O Sapientia (O Wisdom)

O Adonai (O Lord)

O Radix Jesse (O Root of Jesse)

O Clavis David (O Key of David)

O Oriens (O Dayspring)

O Rex Gentium (O King of the Nations)

O Emmanuel (O With Us is God)

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From last to first, the first letters of the titles form a Latin acrostic meaning "Tomorrow I will be there" -

Emmanuel Rex Oriens Clavis Radix Adonai Sapientia

ero cras - Tomorrow I will come